Traditional plano (that is, non corrective or zero power) eyewear is constructed using lenses which are flat or spherical. A spherical lens surface is obtained when a circle is rotated about its diameter. Prior art FIGS. 1 and 2 depict a conventional spherical lens where FIG. 1 is a unitary spherical lens and FIG. 2 is a bispheric lens. As shown in FIGS. 1 and 2, the lens surface is a segment of a sphere such that a cross-section taken in any meridian through the center of the sphere will reveal an arc of constant radius R.
More recently, in an effort to improve protection to the wearer's eyes, attempts have been made to allow the lenses to curve along more of the wearer's face, that is, to achieve a larger wrap depth so as to protect the sides of the eyes. This has been accomplished using toric and cylindrical surfaces, all of which have been based on circular or spherical geometries. A circular toric surface is obtained when a circle is rotated about an axis which is located in the same plane as the circle, but at some distance from the diameter of the circle. The resulting three dimensional shape is a donut or toroid and the toric lens consists of a section of that formed donut. Prior art FIGS. 3 and 4 depict toric lenses where FIG. 3 is a unitary circular toric lens and FIG. 4 is a bi-toric lens. In both FIGS. 3 and 4, the resulting lens has a constant radius of curvature in the horizontal meridian of R.sub.2 and a constant radius of curvature in the vertical meridian of R.sub.1. Prior art examples of toric plano eyewear is disclosed in U.S. Pat. No. 4,867,550 to Jannard and U.S. Pat. No. 4,741,611 to Burns.
A cylinder is obtained when a circle is rotated about an axis which is located an infinite distance from the diameter of the circle. Prior art FIG. 5 depicts a unitary cylindrical lens used in eyewear. The cylinder can also be described as the surface obtained when a circle is extruded in a direction perpendicular to the plane of the circle. As shown in FIG. 5, the resulting lens has a constant radius of curvature R.sub.1 in the horizontal meridian and a vertical axis radius of curvature R.sub.2 equal to .infin. (which is essentially a straight line). Prior art examples of cylindrical plano eyewear are disclosed in U.S. Pat. No. 4,674,851 and U.S. Pat. No. 4,859,048 to Jannard.
As mentioned, all four of the aforementioned lens surfaces (flat, spherical, toric and cylindrical) are based on circular or spherical geometries. Several favorable advantages resulting from the use of such geometries are that the optical performance is easily predictable and the lens surfaces lend themselves easily to manufacture including mold production and lap polishing.
However, the foregoing conventional lens surfaces also suffer from certain serious drawbacks and deficiencies. For example, when spherical, toric or cylindrical lenses are used in piano safety eyewear, it is almost always necessary to have a separate sideshield for lateral protection of the eye. In some commercial designs, the sideshield is a separate component which is attached to, or integrally part of, the temple. In other commercial designs, the sideshield is integrally molded or formed into the lens. In the latter case there is an obvious, visible line of demarcation between the lens, and what is considered to be the sideshield. An example in the prior art of the requirements for such sideshields (either as a separate component or as an integrally molded feature) is described in U.S. Pat. No. 5,381,192 and shown in FIG. 6.
Although lenses based on circular and spherical geometries are easier to produce and their optical properties, easier to predict, design flexibility is limited because the radii in the horizontal and vertical axes are constant. Attempts have been made to design unitary lenses having integrally molded sideshields and no visible line of demarcation between the lens and the sideshield area. However, such lenses (which are made with spherical, cylindrical or circular toric surfaces) will not have sufficient wrap around the sides of the eyes to meet safety standards for lateral protection without being cosmetically and/or functionally unappealing. In order to achieve sufficient wrap, the spectacles have a tendency to take on a "bug-eyed" appearance. The "bug-eyed" appearance can be minimized by utilizing flatter curves, but a flatter curve does not wrap sufficiently close to the temple area. The "bug-eyed" appearance can be somewhat minimized by producing a circular toric. A circular toric lens can be flatter in the vertical meridian but still remains steep in the horizontal meridian. In order to achieve zero power, and to have lens edge thicknesses which will meet safety product impact requirements, and to have sufficient wrap without a separate sideshield, the lens center thickness tends to be relatively high, making the lens heavy and therefore less desirable. An examples of a prior art lens of this type is U.S. Pat. No. 5,032,017 to Bolle et al.
Still another lens surface which provides sufficient wrap but nevertheless maintains an unacceptably large "bug-eyed" appearance for many applications is disclosed in U.S. Pat. No. 4,978,182 to Tedesco. In this latter patent, an ellipse is rotated about its major axis to form an ellipsoid. A section of this ellipsoid is then used to form an eye shield. However, the resultant ellipsoidal lens surface protrudes substantially from the wearer's face and therefore suffers from the same "bug-eyed" appearance as does conventional spherical lens surfaces.